Microwave-assisted synthesis of biodiesel by a green carbon-based heterogeneous catalyst derived from areca nut husk by one-pot hydrothermal carbonization

In this study, we have synthesized a solid acid catalyst by areca nut husk using low temperature hydrothermal carbonization method. The fabricated catalyst has enhanced sulfonic actives sites (3.12%) and high acid density (1.88 mmol g−1) due to –SO3H, which are used significantly for effective biodiesel synthesis at low temperatures. The chemical composition and morphology of the catalyst is determined by various techniques, such as Fourier transform infrared (FTIR), powder X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), Scanning electron microscope (SEM), Energy disruptive spectroscopy (EDS), Mapping, Thermogravimetric analysis (TGA), CHNS analyzer, Transmission electron microscopy (TEM), particle size analyzer, and X-ray photoelectron spectroscopy (XPS). Acid–base back titration method was used to determine the acid density of the synthesized material. In the presence of the as-fabricated catalyst, the conversion of oleic acid (OA) to methyl oleate reached 96.4% in 60 min under optimized conditions (1:25 Oleic acid: methanol ratio, 80 °C, 60 min, 9 wt% catalyst dosage) and observed low activation energy of 45.377 kJ mol−1. The presence of the porous structure and sulfonic groups of the catalyst contributes to the high activity of the catalyst. The biodiesel synthesis was confirmed by gas-chromatography mass spectrometer (GC–MS) and Nuclear magnetic resonance (NMR). The reusability of the catalyst was examined up to four consecutive cycles, yielding a high 85% transformation of OA to methyl oleate on the fourth catalytic cycle.


Scientific Reports
| (2022) 12:21455 | https://doi.org/10.1038/s41598-022-25877-w www.nature.com/scientificreports/ method. Both batches are prepared using 80 °C and 100 °C temperatures. The obtained materials are washed with deionized water until no residues of sulfate appear. The obtained black solid material is dried in the oven overnight. The obtained materials are named SANH18, SANH 28, SANH110, and SANH210. Among all these catalysts, the SANH18 catalyst is used due to highly active sulfonic sites.
Catalyst characterization. Many techniques are applied to characterize the as-synthesized catalyst for elemental and chemical composition. The KBr pellets were examined using a Bruker 3000 Hyperion Microscope equipped with a Vertex 80 FTIR instrument. Powder XRD having Cu-Kα radiation (2θ = 10-90) and a scan speed of 2° min -1 was performed on a Phillips X'pert Pro MPD (multi-purpose diffractometer). SEM-EDS-Mapping (magnification of 10 5× ) was performed on FEI Quanta FEG 200F with Schottky emitter (− 200 V to 30 kV). Gold nanoparticles suspended on carbon substrate while serving HRSEM. Before measuring BET and N 2 adsorption-desorption using a QuantaChrome Nova 2200e Pore Size and Surface Area Analyzer, the material was degassed at 80 °C for 6 h. CHNS analyzer was used to calculate the sulfur content with the help of Elemental Vario EL III. X-ray photoelectron spectroscopy of the sample was examined on PHI 5000 Versaprobe III with dual-beam charge neutralization. To measure the elements C 60 ion gun and argon ion gun and the diameter of sample holders are 25 mm and 60 mm. TGA was measured between 30 and 700 °C in the presence of N 2 gas. The particle size of the material is tested by Zetasizer nano ZSP (ZEN 5600). Joel/JEM 2100 was used for high-resolution transmission electron microscopy (HRTEM) using an electron gun made of LaB6 operating at 200 kV. The acid group density in the catalyst is measured using the acid-base back titration method 22 . The catalyst is mixed with 50 mL of NaOH (0.1 M) solution and stirred for 24 h. The solution obtained after stirring is filtrated and recovered. Then the solution is titrated against NaCl (0.01 M). In this method, phenolphthalein act as an indicator.
Catalyst activity. Homogeneous methanol: OA (10:1-30:1 M ratio) and heterogeneous catalyst (3-9 wt% OA) were added to an ACE pressure tube and heated to 60-100 °C in a microwave for 40-100 min. Rapid and uniform heating of the reaction mixture under microwave radiation has given microwave-assisted biodiesel manufacturing a boost in popularity. The response rates were considerably increased while reaction times were reduced; as a result, this was both economically and environmentally beneficial. This is done by using thin-layer chromatography to monitor the reaction's development. A rotating vacuum evaporator was used to extract the excess MeOH from the solution.
Biodiesel characterization. Using NMR spectroscopy, the esterification product's purity was confirmed and densified. FAME's chemical structure and production were investigated using 1 H and 13 C-NMR on a Bruker Avance III series equipped with a frequency of 500 MHz and TMS as a reference standard. GC-MS was used to determine the FAME's chemical makeup. Injectors with split/splitless technology were used in GC on an Agilent model 8890 with polar columns DB-WAX & HP-5 MS UI Agilent column and split/split injectors. In the beginning, the oven temperature was 50 °C, which rose at a pace of 5 °C/min to 350 °C. An Agilent 5977 MSD apparatus with a mass range of 1.6-1050 amu was used for the MS component of the GC-MS experiment.
Catalyst reusability. After the esterification reaction, the heterogeneous catalyst was filtered using Whatman paper. The catalyst was washed several times with methanol to remove impurities on the surface of the catalyst. The obtained material was dried in a vacuum oven at 80 °C for 4 h. The dried material is used for the next esterification process without any treatment.

Result and discussion
Effect of temperature on hydrothermal carbonization. The effect of temperature on carbonization plays a significant impact in catalyst activity. When the temperature of the carbonization process increases, the lignin, cellulose, and hemicelluloses part of the Areca nut husk is broken down into numerous polycyclic aromatic hydrocarbons through the process of dehydration condensation. In this manner, a skeletal structure is generated that is advantageous for incorporating the sulfonic acid moieties 23 . But a high temperature causes the expansion of the reticular system and an increase in the carbon content resulting in a decrement in sulfonic acid active site groups 24 . Thus, an appropriate temperature is required for the carbonization process to facilitate the sulfonation process.
Effect of sulfonation. The temperature had an influence on the activity and stability of the sulfonic acid introduced into biomass. An unstable and readily decomposable chemical was formed as a consequence of the restricted sulfonation. A decrease in catalytic activity may be caused by an unwanted multi-sulfonated group when temperatures rise too high. Increasing the sulfonation temperature to 100 °C and then 120 °C resulted in a lower S-content 25 . However, the degree of functionalization increases with decreasing carbonization temperature; in other words, materials functionalized at lower temperatures are anticipated to have a more significant density of acid sites. At low carbonization temperatures, the biomass structure is not entirely decomposed. As a result, surface groups are formed that may react with sulfuric acid more efficiently, encouraging the increased incorporation of sulfonic groups on the surface of carbon catalysts 24 . Carbon network breaking, as well as high-temperature acid group dehydration, was accountable for the decrease in sulfonation surface areas.
The poor thermal resilience of -SO 3 H groups can be attributed to the unfavorable influence on the S content of the catalyst at high reaction temperatures 26 26 . Figure 1 shows the synthesis of the catalyst by hydrothermal carbonization and the formation of active sites, which are responsible for the esterification process. Table 1 describes the total sulfur present analyzed by CHNS analyzer and the acid density of the synthesized catalysts.
FTIR. FTIR spectrum of the SANH18 catalyst is shown in Fig. 2. Numerous sulfonic acid, hydroxyl, and carbonyl functional groups should be present in an efficient and effective heterogeneous acid catalyst associated with carbonaceous material. FTIR spectrum is used to analyze weak or strong acidic groups, which act as  www.nature.com/scientificreports/ noteworthy active sites for esterification reactions by synergistic effect. The high intensity of the 1028 cm -1 peak in the SANH18 catalyst corresponds to the symmetric stretching of -SO 3 H, indicating the formation of a sulfonated catalyst 27 . Our results show the successful sulfonation of biomass-based catalysts even at low temperatures. Zhang et al. claim that the -SO 3 H group replaces hydrogen on the surface of a solid and attaches covalently to carbon 28 . The spectra revealed that the C=C and carbonyl stretching in the aromatic rings were responsible for the 1605 cm -1 and 1656 cm -1 peak. Peaks 2853-3246 cm -1 pertaining to the aldehydes and sp 3 C-H groups.
In the 3359-3756 cm -1 region, the O-H stretching modes are clearly visible 29 . This indicates that along with the strong -SO 3 H group, there is the existence of week -OH and -COOH groups. Figure 3 shows the appropriate XRD spectrum of as-fabricated SANH18 catalysts with the greatest sulfur concentration. The catalyst has a wide peak at 2θ = 18-28°, indicating its amorphous nature, in which carbon are positioned arbitrarily 30 . In the SANH18 catalysts, the amorphous phase displays a wider amorphous hump. Polycyclic aromatic carbon sheets are bounded by randomly arranged -SO 3 H, -OH and -COOH groups 31 .

Morphological and elemental composition analysis.
To study the morphology of SANH18 catalyst, SEM and TEM micrographs are taken. The SEM images show the mesoporous surface of the SANH18 catalyst, which is aggregated with different elements which consist of varying particle sizes (Fig. 4a-c). The carbonaceous sheetlike frameworks were detected by TEM (Fig. 4d-f) in the SANH18 catalyst; however, their formation was incomplete 27 . TEM images show that carbonaceous materials exist in the aromatic sheets. Particle size analysis data indicates that the material size ranges in micrometers which is consistent with the TEM results. From the result, it was found that the particle size of the material ranges between 220 to 712 nm. The highest peak of 391.9 nm was observed for the carbonaceous material (Fig. 5f). Figure 5e depicts the EDS analysis used to identify the components in the catalyst, and it reveals that carbon (73.72%), oxygen (22.97%), and sulfur (3.31%) are present. The confirmation of high sulfur was closely associated with CHNS analyzer as shown in Table 1. Higher sulfur content in SANH18 was shown to have outstanding catalytic activity in the current investigation. In the form of sulfonic acid groups, sulfur was the principal active ingredient, contributing to the performance and activities of the fabricated catalysts. Covalent bonds between sulfuric acid and the catalyst result in greater acid capacity. Mapping of the catalyst shows a close association between C, O, Si, and S and is clearly homogenized and evenly distributed after the extraction (Fig. 5a-d).
Further evidence of the presence of the active species on the catalyst surface was provided by the homogeneous dispersion of sulfur elements. SANH18 had the most incredible sulfur content because of its wide specific surface area and high content of sulfonating agent, both of which contributed to its high sulfur loading. Due to a more considerable extent in the augmentation of surface area and pore volume induced by side reactions such as oxidation, the vast proportion of sulfonating agents resulted in an improved structure of the carbon framework 32 .

BET.
To further understand the surface of SANH18 catalyst, pore size distribution and N 2 adsorption-desorption isotherm were conducted (Fig. 6a,b). The N 2 isotherm curve, as in Fig. 6b graphical representation, shows the Type-IV hysteresis loop followed the mesoporous surface as no pore opened at both ends 33 . The data of N 2 adsorption-desorption clearly match the mesoporous surface (d = 2-50 nm) with the SEM images 34 . To find the pore structure, the BET technique was used to measure the pores' diameter, volume, and surface area.  It wasn't only pore shape and surface area that affected esterification reaction catalytic activity. Faster reaction rates may be achieved using mesoporous materials, allowing the reactants to spread into the pores. Whereas in a microporous surface, the reaction occurs at the opening of the pores, resulting in lower reaction rates 35 .

XPS.
The chemical valences of the interface group on SANH18 were investigated using XPS characterization.
XPS study confirms the presence of carbon, oxygen, and sulfur. The finding of XPS is consistent with EDS analysis (Fig. 5e), indicating the successful sulfonation of biomass at low temperatures. The data of carbon, oxygen, and sulfur is shown in Fig. 7. At 286 and 288 eV, the C 1s spectra displayed two peaks. Carbon atoms bonded with sulfur in sulfonic moieties may be allocated to the prominent peak positioned at 286 eV 36 . A C-C and a C=O link in carboxylic groups, both of which have peaks at 286 and 288 eV spectral energies, further demonstrate the existence of oxygen groups 37,38 . Moreover, the peak in the oxygen spectrum at 531 and 533 eV is ascribed to C-OH and C=O bond 39 . Notably, the S 2p area exhibited two peaks at 170 and 171 eV, was ascribed to a high oxidation state of sulfur in -SO 3 H, which is in accordance with FTIR data. The findings from the XPS experiment make it abundantly evident that SANH18 was successfully sulfonated, and they lend credence to the theory that any visible sulfur in the catalyst is present as sulfonic acid groups. Thermogravimetric analysis. TGA analysis was used to assess the SANH18 catalyst's thermostability. Figure 8 showing TGA thermographs. The primary mass loss of 11.4% SANH18 catalyst up to 200 °C was attributed absorbed water and other volatile substances. In the next phase of 200 °C to 500 °C, the degradation of organic substances occurs such as proteins, lipids and carbohydrates 40 . As a result, a highest mass loss of 29.4% was observed followed by a mass loss of 8.2% at 700 °C due to carbonaceous materials decomposition and releasing CO 2 and CO etc. 41,42 . Some studies indicate that materials decomposition starts from 420 °C and release CO 2 and CO 43 .

Esterification reactions
Esterification reactions were carried out utilizing methanol and oleic acid as well as a catalyst based on sulfuric acid. The study was conducted using a 15 mL glass-sealed tube at 80 °C for 1 h. For esterification reactions, a molar ratio of methanol and oleic acid (15:1, 20:1, 25:1, 30:1) is analyzed. It is clear that as the sulfonating agent concentration was increased, the FAME production also climbed. This is due to the additional sulfonic acid groups that have been integrated into the catalyst surface, giving it enhanced catalytic activity for the esterification step. The FAME conversion was 96.4%, indicating that the synthesized catalysts had promising catalytic activity in the production of biodiesel.
The catalyst dose has a significant impact on the yield of the generation of biodiesel and the rate of reaction. With a catalyst dose of 9 wt%, 96.4% of biodiesel production was achieved, indicating a steep slope in the outcomes (Fig. 9a). Beyond increasing the catalyst dosage, there was no consistent change in the FAME production, indicating that greater doses had no advantages over lower dosages due to increased viscosity and catalytic site saturation 44,45 . As a result, more active sites were accessible to complete the catalytic reaction. As a result, the ideal catalyst dose was determined to be 9 wt%. When contrasted with other acidic catalysts, the synthesized SANH18 demonstrated high efficacy with reduced catalyst loading (9 wt%) [46][47][48][49] .
The transformation of the feedstock into FAME may be influenced by the molar ratio of oleic acid to methanol. Figure 9b shows that when the stoichiometric ratio was adjusted from 10 to 30, a substantial improvement was realized. At a molar ratio of 25, the most significant FAMEs yield (96.4%) was achieved (Fig. 9b). However, when www.nature.com/scientificreports/ the molar ratio was increased to 30, the conversion decreased somewhat. Lower conversion may be caused by partial blocking of the catalyst's active sites due to an overabundance of methanol 50 . The esterification reactions are reversible in nature, so additional alkyl donors are required with respect to oleic acid to reach a conclusion. The reverse reaction can also be decreased due to excess methanol. It was thus decided to proceed to the following optimization phase with a molar ratio of 25. As the molar ratio of methanol to oleic acid increased, so did the catalyst's availability for interaction with the oleic acid. Because of this, the reaction system had a smaller collision frequency with the catalyst, reducing the mass transfer effect 51 .  www.nature.com/scientificreports/ Temperature is crucial for studying FAME production because it helps reach the activation energy required for the esterification reaction. It is conceivable that an ideal temperature of the reaction must be determined to get a reasonable yield while maintaining a sustainable operating cost. For biodiesel production, temperatures ranged from 60 to 90 °C (Fig. 9c). At the low temperature, the activity was sluggish-the high-temperature results in low viscosity, higher collision frequency, higher diffusion rate, and better solubility. For example, the biodiesel conversion increases as the reaction temperature rise from 70 to 80 °C, reaching 96.4% at 80 °C. The biodiesel output drops to 94.1% at temperatures over 90 °C. To improve interactions and miscibility that lead to bond breaking and cleavages, the reaction temperature makes it easier for molecules of reactants to collide 52 . Uncontrolled vaporization may be to blame for the drop in biodiesel output at 90 °C, which reduces the amount of accessible methanol and, thus, the number of reactive species required for the esterification process 53,54 .
Time is an essential factor in biodiesel synthesis since it affects the activity of the catalyst 55 . As described in Fig. 9d, the time is varied from 40 to 100 min for biodiesel production. The optimum biodiesel conversion was obtained in just 60 min. On further increments of time, there is little change in the conversion of biodiesel. This may be explained by the bidirectional esterification process, which can be induced by prolonging the response time beyond its excellent value. It then ended in the hydrolysis of the biodiesel generated 56 . According to research, a lengthy response time might reduce the surface area by lowering the active sites 57,58 . In 60 min, the maximum amount of biodiesel conversion was obtained (96.4%) and showed the complete utilization of oleic acid. Table 2 summarizes the various parameter effects on the esterification process.

Kinetic analysis
A high concentration of methanol in the esterification reaction postulates the pseudo-first-order reaction 59 . The oleic acid methanolysis reaction takes place in the homogeneous regime by SANH18 catalyst, where the overall reaction rate is determined by chemical reactions. The perceptible rate constant of reaction (k) was determined from the slope of -ln(1−x) vs. reaction time. The obtained linear line indicates the high value of R 2 (0.97-0.99) confirms the pseudo-first-order reaction 60 and is used for elucidation of the kinetics of SANH18 (Fig. 10a).
The activation energy was calculated using the Arrhenius equation (Eq. 1) and rate constants at various temperatures (50-80 °C). www.nature.com/scientificreports/ of characteristics peak at 3.665 ppm corresponding to methoxy protons was compatible with the synthesis of methyl esters (Fig. S2). The further confirmation of methyl ester formation is done with the help of 13 C-NMR, which shows a signal at 51.435 ppm indicates the formation of methyl esters (Fig. S3). Equation (2) is used to calculate the percentage of conversion yield of biodiesel.

Catalyst stability and reusability
A suitable catalyst shows high catalytic activity and stability even after several uses. To check the reusability of the SANH18 catalyst, we successfully ran a batch esterification reaction of 1 h in the microwave under optimized conditions (25:1 methanol/OA ratio, 80 °C, 9 wt% catalyst). The catalyst used was washed with methanol with the help of filtration. The subsequent cycle of 1st, 2nd, 3rd, and 4th shows excellent free fatty acid (FFA) conversion of 96.4%, 92%, 87%, and 85%, respectively, as shown in Fig. 11. But after 4 consecutive cycles, the transformation of FFA drops to 75%. According to the literature, this may be due to hydrocarbon residue on the surface of the catalyst, byproducts of the reaction, water adsorption, and reduction in active sulfonic sites 64 . As water is also a byproduct of esterification reaction, so it reduces the activity by acid site contaminations. In the majority of the literature survey, this may be due to the deactivation of catalyst by leaching of SO 3 H groups 27,65 . EDS spectra (Fig. S4b) of reused catalyst reveals the decrease in wt% of sulfur (3.31 wt% to 2.17 wt%). FTIR data (Fig. S4a) of reused also reveals the involvement of SO 3 H groups as the slight peak shifts of these sulfonic groups.

Conclusion
The batch reactions of solid acid catalyst based on Areca nut husk as raw material was synthesized using a onepot hydrothermal carbonization method at just 80 °C. The sulfuric acid concentration and temperature have a significant effect on the acid density of the SANH18 catalyst. Due to sulfuric acid strength, it causes the release of H + ions, allowing fatty acids carboxylic moieties to be protonated. The carbonaceous catalysts possess high sulfonic groups, performed exceptionally well in the esterification process. The fine-tuned parameters provided better textural qualities regarding total acidity and specific surface area. The maximum esterification by OA was found to be 96.4% under the optimization process (9 wt% catalyst dosage, 25:1 methanol: OA ratio, 80 °C) after 1 h, which is excellent compared to other studies. This stipulates that this acid catalyst is used in many acid-functionalized reactions. The kinetic analysis of the reaction follows the pseudo-first-order due to the high concentration of methanol. The SANH18 catalyst, in the presence of SO 3 H active species, shows perfect activity for the esterification process. The sulfonated catalyst offers high reusability up to four cycles. Moreover, the conversion of FFA into FAME decreases during the reusability test due to humins deposition on the catalyst surface. As a result, the heterogeneous SANH18 catalyst is an effective material for producing economically and environmentally friendly biodiesel. As areca nut husk is found in large quantities in the region, all of the husks go to waste. The catalyst obtained from areca nut husk has a lot of potential to solve environmental problems and is used for sustainable fuel synthesis. The catalyst is particularly successful in early scale-up operations since it is made from a low-cost, easily accessible, and readily renewable kind of biomass, and it also lowers the cost of biodiesel production at an industrial scale.

Data availability
The authors declare that all data supporting the findings of this study are available within the article and its supplementary information files.